Move prefer_related_applications and related_applications to manifest incubation

[christianliebel]

Closes #956, closes #957

This change (choose at least one, delete ones that don't apply):

• Breaks existing normative behavior (please add label "breaking")
• Adds new normative requirements
• Adds new normative recommendations or optional items
• Makes editorial changes (changes informative sections, or changes normative sections without changing behavior)
• Is a "chore" (metadata, formatting, fixing warnings, etc).

Implementation commitment (delete if not making normative changes):

• WebKit (https://bugs.webkit.org)
• Chromium (https://bugs.chromium.org/)
• Gecko (http://bugzilla.mozilla.org)

If change is normative, and it adds or changes a member:

• updated JSON schema

Commit message:

Move prefer_related_applications and related_applications to manifest incubation

Person merging, please make sure that commits are squashed with one of the following as a commit message prefix:

• chore:
• editorial:
• BREAKING CHANGE:
• And use none if it's a normative change

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